The present invention relates to a data reproduction control method and apparatus that control a data reproduction characteristic based on the distribution characteristic of quantized data obtained by quantizing a signal reproduced from a recording medium.
The present invention further relates to an optical disk unit that operates in accordance with such a data reproduction control method.
Recently, optical disk units, magneto-optical disk units, and magnetic disk units that are employed as auxiliary storages of a computer have required a data recording and reproduction method with higher accuracy in order to meet a demand for increase in the recording density of a recording medium.
Conventionally, as a method of recording data on and reproducing data from optical disk units, magneto-optical disk units, and magnetic disk units with high accuracy, the PRML (partial response maximum likelihood) technique has been proposed. By this PRML technique, a recording data signal is modulated into a so-called partial response (PR) waveform to be recorded on a recording medium, while a reproduced signal from the recording medium is sampled at regular intervals to be quantized so that the quantized data is subjected to waveform equalization to correspond to the partial response (PR) waveform. The reproduced signal may be sampled at regular intervals and quantized after being waveform-equalized to correspond to the partial response (PR) waveform. Then, from the resulting quantized data, data of maximum likelihood is determined in accordance with a maximum likelihood (ML) detection technique such as the so-called Viterbi detection.
The conventional PRML technique employs a(n) (adaptive equalization) technique that controls the characteristic of waveform equalization so as to minimize a difference between quantized data obtained by sampling and a value that is expected to be obtained from a partial response (PR) waveform. By thus controlling the characteristic of waveform equalization, quantized data corresponding to a signal waveform more similar to the partial response (PR) waveform can be obtained. Consequently, data reproduction can be performed with more accuracy.
However, such a technique to control the characteristic of waveform equalization as described above controls the equalization characteristic based on a local state of the reproduced signal. Therefore, if the reproduced signal varies locally due to a noise, stable and more proper data reproduction cannot be achieved.
Further, in order to realize high-density data recording, a medium having a magnetically induced super resolution effect (an MSR (Magnetic Super Resolution) medium), for instance, is about to be put into practical use. In this MSR medium, a super resolution effect is produced by forming a mask using the heat distribution of a light beam, so that a nonlinear distortion component is generated in the reproduced signal by a non-uniform heat distribution of the light beam moving on the medium. This nonlinear distortion component distorts the reproduced waveform.
This distortion appears as a droop phenomenon that occurs notably at a trailing or rising edge of the reproduced signal. Further, this distortion is not generated only at a given point, but is generated steadily.
Thus, there has been proposed a technique to reproduce two clock signals synchronizing with the trailing and rising edges of the reproduced signal, respectively, (PLL) and sample the reproduced signal by using the two clock signals (DUAL PLL system).
Even if the reproduced signal is distorted at a trailing or rising edge thereof, sampling can be performed at a timing corresponding to the distortion by sampling the reproduced signal by such clock signals synchronizing with the trailing and rising edges of the reproduced signal, respectively. Therefore, correct output data can be obtained.
However, the above-described DUAL PLL system has a complicated circuit structure because of its employment of the two clock signals to sample the reproduced signal. On the other hand, there is another method of sampling the reproduced signal which method employs a clock signal that facilitates circuit simplification (SINGLE PLL system). However, the SINGLE PLL method is prevented from obtaining correct output data in the case of sampling such a drooping waveform as described above.
It is a general object of the present invention to provide a useful data reproduction control method and apparatus, and a useful optical disk unit that are improved to eliminate the above-described disadvantage.
An object of the present invention is to provide a data reproduction control method and apparatus that can reproduce data stably and more properly with as simple a circuit structure as possible even when a reproduced signal varies locally due to a noise to be distorted or includes a steady distortion at a trailing or rising edge thereof.
A more specific object of the present invention is to provide a data reproduction control method and apparatus that can reproduce data stably and more properly even when a reproduced signal varies locally due to a noise.
The above objects of the present invention are achieved by a data reproduction control method controlling a distribution characteristic of quantized data by adjusting a waveform equalization characteristic at a time of performing quantization and waveform equalization on a signal reproduced from a recording medium recorded with data in accordance with a given recording code, wherein frequencies of levels of the quantized data are counted, and the waveform equalization characteristic is adjusted so that a distribution of the counted frequencies of the levels approaches a distribution that should be obtained based on the given recording code.
The above objects of the present invention are also achieved by a data reproduction control apparatus controlling a distribution characteristic of quantized data by adjusting a waveform equalization characteristic of a waveform equalizer when a signal reproduced from a recording medium recorded with data in accordance with a given recording code is subjected to operations in quantization means and the waveform equalizer, which data reproduction control apparatus includes counting means for counting frequencies of levels of the quantized data, and equalization characteristic adjustment means for adjusting the waveform equalization characteristic of the waveform equalizer so that a distribution of the frequencies of the levels obtained in the counting means approaches a distribution that should be obtained based on the given recording code.
According to the above-described method and apparatus of the present invention, by setting a period of signal reproduction relatively long, the frequencies of the levels of quantized data are counted based on a larger amount of quantized data. Waveform equalization characteristic is adjusted based on a distribution of the thus obtained frequencies of the levels of the quantized data. As a result, the waveform equalization characteristic is adjustable so that a more proper waveform equalization output can be obtained even if a reproduced signal varies locally.
In another mode of the present invention, the frequencies of the levels of the quantized data may be counted at given intervals in the period of signal reproduction from the recording medium.
In yet another mode of the present invention, a given number of periods of signal reproduction from the recording medium may be set, and the frequencies of the levels of the quantized data may be counted and the waveform equalization characteristic may be adjusted in each of the periods of signal reproduction.
In another mode of the present invention, the waveform equalization characteristic may be adjusted by a given amount in each of the periods of signal reproduction.
In yet another mode of the present invention, the waveform equalization characteristic may be adjusted by a first given amount in first given ones of the periods of signal reproduction, and be adjusted by a second given amount less than the first given amount in the rest of the periods of signal reproduction.
In yet another mode of the present invention, a discrete state in the distribution of the counted frequencies of the levels may be determined, and the waveform equalization characteristic may be adjusted so that the discrete state may match a discrete state of the distribution that should be obtained based on the given recording code.
Yet another object of the present invention is to provide a data reproduction control method and apparatus that realizes stable and more proper data reproduction even if a reproduced signal includes a steady distortion.
The above objects of the present invention are achieved by a data reproduction control method performing quantization on a signal reproduced from a recording medium recorded with data in accordance with a given recording code, and controlling a data reproduction characteristic at a time of determining, by a maximum likelihood detection process corresponding to the recording code, reproduced data from quantized data obtained by the quantization, wherein frequencies of levels of the quantized data are counted, and a processing characteristic in the maximum likelihood detection process is adjusted based on a distribution of the counted frequencies of the levels.
In another mode of the present invention, in the above-described data reproduction control method, the maximum likelihood detection process may be a Viterbi decoding algorithm, and expected values employed in the Viterbi decoding algorithm may be provided based on the distribution of the counted frequencies of the levels.
According to this data reproduction control method, even if a reproduced signal includes a steady distortion, an operation following a maximum likelihood detection process is performed with a processing characteristic matching the distortion. Therefore, even if the reproduced signal includes a distortion, stable and more proper data reproduction can be realized.
In yet another mode of the present invention, in the above-described data reproduction control method, a trailing edge of the reproduced signal may be detected based on a level change of the quantized data, the frequencies of the levels of the quantized data may be counted at the detected trailing edge of the reproduced signal, and the processing characteristic in the maximum likelihood detection process for the trailing edge of the reproduced signal may be adjusted based on the distribution of the counted frequencies of the levels.
In yet another mode of the present invention, in the above-described data reproduction control method, a rising edge of the reproduced signal may be detected based on a level change of the quantized data, the frequencies of the levels of the quantized data may be counted at the detected rising edge of the reproduced signal, and the processing characteristic in the maximum likelihood detection process for the rising edge of the reproduced signal may be adjusted based on the distribution of the counted frequencies of the levels.
According to these data reproduction control methods, when a reproduced signal includes a distortion especially at its trailing or rising edge due to a characteristic of a recording medium, a processing characteristic in a maximum likelihood detection process for the edge is individually adjustable.
The above objects of the present invention are also achieved by a data reproduction control apparatus performing quantization on a signal reproduced from a recording medium recorded with data in accordance with a given recording code, and controlling a data reproduction characteristic when maximum likelihood detection means, in accordance with a maximum likelihood detection process corresponding to the recording code, determines reproduced data from quantized data obtained by the quantization, which data reproduction control apparatus includes counting means for counting frequencies of levels of the quantized data, and characteristic adjustment means for adjusting a processing characteristic in the maximum likelihood detection means based on a distribution of the frequencies of the levels obtained in the counting means.
In another mode of the present invention, in the above-described data reproduction control apparatus, the maximum likelihood detection means may be Viterbi detection means by which the reproduced data is determined in accordance with a Viterbi decoding algorithm, and the characteristic adjustment means may include expected value setting means for setting expected values employed in the Viterbi detection means based on the distribution of the counted frequencies of the levels.
In yet another mode of the present invention, the above-described data reproduction control apparatus may include trailing edge detection means for detecting a trailing edge of the reproduced signal based on a level change of the quantized data, and counting control means for enabling the counting means while the trailing edge detection means detects the trailing edge of the reproduced signal, wherein the characteristic adjustment means may adjust the processing characteristic in the maximum likelihood detection means for the trailing edge of the reproduced signal based on the distribution of the frequencies of the levels obtained in the enabled counting means.
In yet another mode of the present, invention, the above-described data reproduction control apparatus may include rising edge detection means for detecting a rising edge of the reproduced signal based on a level change of the quantized data, and counting control means for enabling the counting means while the rising edge detection means detects the rising edge of the reproduced signal, wherein the characteristic adjustment means may adjust the processing characteristic in the maximum likelihood detection means for the rising edge of the reproduced signal based on the distribution of the frequencies of the levels obtained in the enabled counting means.
Yet another object of the present invention is to provide an optical disk unit that operates in accordance with the above-described data reproduction control methods.
The above object of the present invention is achieved by an optical disk unit having a reproduction system including processing means for performing quantization and waveform equalization on a signal reproduced from an optical disk medium recorded with data in accordance with a given recording code, and maximum likelihood detection means for determining, in accordance with a maximum likelihood detection process corresponding to the recording code, reproduced data from waveform-equalized quantized data obtained in the processing means, which optical disk unit includes counting means for counting frequencies of levels of the quantized data, and equalization characteristic adjustment means for adjusting a waveform equalization characteristic in the waveform equalization so that a distribution of the frequencies of the levels obtained in the counting means approaches a distribution that should be obtained based on the given recording code.
The above object of the present invention is also achieved by an optical disk unit having a reproduction system including quantization means for performing quantization on a signal reproduced from an optical disk medium recorded with data in accordance with a given recording code, and maximum likelihood detection means for determining, in accordance with a maximum likelihood detection process corresponding to the recording code, reproduced data from quantized data obtained in said quantization means, which optical disk unit includes counting means for counting frequencies of levels of the quantized data, and characteristic adjustment means for adjusting a processing characteristic in the maximum likelihood detection means based on a distribution of the frequencies of the levels obtained in the counting means.